Muscular system - class-XI

Extensor muscles. Attached to the back of the spine, these muscles allow us to stand and lift objects. They include the large muscles in the lower back (erector spinae), which help hold up the spine, and gluteal muscles.

The stapedius muscle, which is innervated by the facial nerve (cranial nerve VII), is the smallest of all skeletal muscles.

The gluteus maximus is the largest muscle in the human body. These muscles help move the hips and thighs and also keep the trunk of the body in upright position. These muscles work against the gravity when we are walking up stairs. So, the correct answer is 'Gluteus maximus'.

Smooth muscles (found in organs) and cardiac muscles (found in the heart) are involuntary. Striated (skeletal) muscles are voluntary.

• Sarcoplasm is the cytoplasm of a myocyte (muscle fibre, muscle cell).
•  It is comparable to the cytoplasm of other cells, but it contains unusually large amounts of glycosomes (granules of stored glycogen) and significant amounts of myoglobin, an oxygen-binding protein. Hence, the Cytoplasm of a muscle cell is sarcoplasm.

• Myoglobin is an iron- and oxygen-binding protein found in the muscle tissue of vertebrates in general and in almost all mammals. 
• It is distantly related to haemoglobin which is the iron- and oxygen-binding protein in blood, specifically in the red blood cells. Hence, Muscles contain a red coloured oxygen-storing pigment called myoglobin.

• Muscle contains a red coloured oxygen-containing pigment called myoglobin. It is similar to hemoglobin pigment that stores oxygen in the blood cells. Myoglobin binds oxygen with its heme group.

1. sarcoplasmic reticulum is a specialized type of smooth ER that regulates the calcium ion concentration in the cytoplasm of striated muscle cells. 
   
3. sarcoplasmic reticulum  is a membrane-bound structure found within muscle is also known as endoplasmic reticulum of muscle. so option A is correct answer .

• The ‘A’ and ‘I’ bands are arranged alternately throughout the length of the myofibrils.
  
• In the centre of each ‘I’ band, there is an elastic fibre called ‘Z’ line which bisects it. The thin filaments are firmly attached to the ‘Z’ line. 
• The portion of the myofibril between two successive ‘Z’ lines is considered as the functional unit of contraction and is called sarcomere.
• Hence Sarcomere is the unit of contraction.
  

• Muscle is a  bundle of fibrous tissue in a body that has the ability to contract, producing movement in or maintaining the position of parts of the body.
  
• The total number of muscles in the human body comes in the range of 640-850.
  
• So,the correct answer is '640-850'

Movement is performed by muscular tissues. Every bone moves with the help of 2 muscles required to move it. 

•  Syncytial or syncytium is a multinucleated cell that can result from multiple cell fusions of uninuclear cells (i.e., cells with a single nucleus), which can result from multiple nuclear divisions without accompanying cytokinesis. 
• Syncytial is also referring to cells interconnected by the specialized membrane with gap junctions, as seen in the heart muscle cells and certain smooth muscle cells, which are synchronized electrically in an action potential. Hence, Condition of having a number of nuclei in a muscle fibre is syncytial.

Answer is option C i.e. "Sacroplasmic reticulum"
Neurotransmitter acetylcholine generates an action potential in the sarcolemma causing the release of calcium ions into the sarcoplasm from the sarcoplasmic reticulum.
When Ca++ level increases it leads to the binding of calcium with a sub-unit of troponin on actin(thin) filaments and thereby activating the movement.

Myoglobin is the iron and oxygen-binding protein that is found in the muscle tissue of the vertebrates and in almost all mammals. This contains heme, the pigment responsible for the red color of meat.

Sarcomere is the structural and functional unit of a myofibril and consists of thick myosin filaments and thin actin filaments. If the allele for actin filament continues to synthesize linear chains of the protein, sarcomere structure would be same as normal. Thus, the correct answer is option C.

• Muscle contraction thus results from an interaction between the actin and myosin filaments that generates their movement relative to one another. 
• The molecular basis for this interaction is the binding of myosin to actin filaments, allowing myosin to function as a motor that drives filament sliding. Hence, The binding of troponin to calcium in muscle cells exposes the binding site of Myosin.

ATPase needed for muscle contraction is present over the myosin. Myosin binds to actin at a binding site. Myosin has another binding site for ATP at which enzymatic activity of myosin ATPase hydrolyzes ATP to ADP which releases an inorganic phosphate molecule and energy. The energy released during ATP hydrolysis changes the angle of the myosin head into a cocked position. The myosin head is then in a position for further movement possessing potential energy. The enzyme myosin ATPase catalyses the reaction in the presence of Ca$^{2+}$ and Mg$^{2+}$. So, the correct answer is 'Myosin'.

• Active sites for myosin(Thick filament) are present on actin which are masked by troponin in resting state. when  Ca++ level increases it leads to the binding of calcium with a subunit of troponin on actin(thin) filaments and thereby remove the masking of active sites for myosin.
• Utilizing the energy from ATP hydrolysis, the myosin head now binds to the exposed active sites on actin to form a cross bridge. 
• This pulls the attached actin filaments towards the centre of ‘A’ band.
•  The ‘Z’ line attached to these actins are also pulled inwards thereby causing a shortening of the sarcomere, i.e., contraction.
• Magnesium also plays a role in regulating muscle contractions. Magnesium acts as a natural calcium blocker to help muscles relax. Hence Ca++ and Mg++ ions are involved in muscle contractions.
• So, the correct answer is 'Ca$^{2+}$ and Mg$^{2+}$'.

The excess ATP is used to synthesize creatine Phosphate, an energy rich molecule that is found in muscle fibres.

•  A neural signal reaching this junction releases a neurotransmitter (Acetylcholine) which generates an action potential in the sarcolemma. This spreads through the muscle fibre and causes the release of calcium ions into the sarcoplasm
  
• Active sites for myosin are present on actin which are masked by troponin-C in resting state. When Ca++ level increases it leads to the binding of calcium with a subunit of troponin i.e Troponin-C on actin(thin) filaments and thereby remove the masking of active sites for myosin.
  
• Utilizing the energy from ATP hydrolysis, the myosin head now binds to the exposed active sites on actin to form a cross bridge. 
• This pulls the attached actin filaments towards the centre of ‘A’ band.
  
• The ‘Z’ line attached to these actins are also pulled inwards thereby causing a shortening of the sarcomere, i.e., contraction. The myosin, releasing the ADP and P1 goes back to its relaxed state. Hence Ca$^{2+}$ is required for muscle contraction and nerve impulse transmission.
• So, the correct answer is 'Ca 2+'.

• Active sites for myosin(Thick filament) are present on actin which are masked by troponin in resting state. when  Ca++ level increases it leads to the binding of calcium with a subunit of troponin on actin(thin) filaments and thereby remove the masking of active sites for myosin.
• Utilizing the energy from ATP hydrolysis, the myosin head now binds to the exposed active sites on actin to form a cross bridge. 
• This pulls the attached actin filaments towards the centre of ‘A’ band.
•  The ‘Z’ line attached to these actins are also pulled inwards thereby causing a shortening of the sarcomere, i.e., contraction.
• magnesium acts as a natural calcium blocker to help muscles relax. Hence Mg ion is not involved in the muscular contraction but it is involved in muscle relaxation by blocking calcium ions.
• So, the correct answer is 'Mg'

Cross bridge cycling forms the basis for movements and force production in the muscle cells. The myosin heads are known as cross bridges because they can bind and move along actin filament. Sarcomere is a stretch of myofibril between two z lines where myosin is in center and actin at periphery, slightly overlapping myosin. Nerve impulse causes release of calcium ions from sarcoplasmic reticulum and binds to troponin which changes configuration of tropomyosin and exposes cross bridge heads on myosin allowing actin myosin binding. This causes contraction of muscle fibre. ATP is required for myosin to dissociate from actin. The calcium ions released from sarcoplasmic reticulum regulate when cross bridging cycle can occur. 

Calcium is the ion released into the sarcoplasma from sarcoplasmic reticulum during the polarisation.  Ca++ attaches to the Troponin-C. This brings a confirmational change in the tropomyosin. As a result unmasking of active sites on actin for myosin takes place. Cross bridges are formed between actin and myosin. This results in muscle contraction.

• The muscle contraction cycle is triggered by calcium ions binding to the protein complex troponin, exposing the active-binding sites on the actin. 
• ATP can then attach to myosin, which allows the cross-bridge cycle to start again; further muscle contraction can occur. 
• Cardiac myosin activators stimulate myosin ATPase, thereby increasing force generation. Hence, Calcium ions bring about muscle contraction
• through Activation of myosin ATP-ase.

During the strenuous muscular activity, hydrogens removed from sugar molecules are accepted temporarily by pyruvate to form lactic acid.

Upon stimulation of skeletal muscles, calcium is immediately made available for binding to troponin from 'sarcoplasmic reticulum'.